Systematic investigation of layered Li (Ni, Mn, Co)O-2 cathode materials was performed to elucidate how Co and Li contribute to the electrochemical properties from electronic and structural points of view. Ni, Mn, and Co K-edge X-ray absorption near-edge structure (XANES) study showed that the Ni valence state was controlled not only by Mn content but also by Co and Li contents in Li1+yNixMnxCo1-2xO2 (0 < x <= 0.5 and 0 <= y <= 0.15) series. Li has larger influence on the Ni valence than Co does. In extended X-ray absorption fine structure (EXAFS) curve-fitting analysis, shortening of Ni - O bond length was observed as y increased (excess Li increased) or x decreased (Co increased), which is also attributed to Ni oxidation. In contrast to Ni, Mn and Co valence states and local structures showed no significant differences on changing the metal content. This study revealed that Li contributes to the rate capability through the concentration of Ni3+ in the transition metal layers, which is closely related to the powder volume resistivity of the active material. Co contributes to the Li-ion conductivity through less cation mixing than to the electronic conductivity of the pristine powder. (C) 2006 The Electrochemical Society.